Diet Percentages: Part 1

Commonly, when you see diet plans laid out, the intake of the various macronutrients (protein, carbohydrate, fat) is presented in terms of percentages of total caloric intake. So you might see a diet which was 60% carbohydrates, 30% protein and 10% fat or some other set of percentages. Or you’ll see recommendations that ‘…athletes only need 15% of their calories from protein.’ or ‘don’t eat more than 30% of your total calories from fat’, that sort of thing.

In this article, I want to teach readers what these percentages mean and how to use them (if you so desire) either analyze a given diet, set up a diet, or figure out what a food label means.

A quick recap on calories

In a previous chapter I gave you the caloric content of the various macronutrients. To save you needless paging, I’ll review them here.

Protein: 4 calories/gram

Carbohydrate: 4 calories/gram

Fat: 9 calories/gram

Alcohol: 7 calories per gra

Calculating percentages

With the above values in hand, and using some basic math, we can do several different operations in terms of diet and food analysis. Let’s look at each one in turn. I’ll give examples but don’t read too much into the numbers. They are only examples.

Operation 1: Setting up diets based on percentages

Probably the most common use of methods is to do actual diet set up, to determine how many grams of each nutrient someone will be consuming. Let’s say we have a 170 pound male with a maintenance calorie level of roughly 2700 calories per day and let’s say we wanted to put him on a diet that was 60% carbohydrate, 20% protein and 20% fat (again, don’t read too much into these values, I’m using them for example only). We want to find out how many grams of each nutrient he will be consuming per day.

Step 1: Calculate total calories of each macronutrient

The first thing we’d do is multiply his total caloric intake (2700 cal/day) by the percentages of each macronutrient as this will tell us how many calories will be coming from each nutrient. To convert percentages, just divide by 100 so 20% becomes 0.20, 60% becomes 0.60, etc.

The calculations appear below

Carbohydrate: 2700 * 0.60 = 1620 calories from carbohydrate

Protein: 2700 * 0.20 = 540 calories from protein

Fat: 2700 * 0.20 = 540 calories from fat

Note: It should be obvious that the percentages need to total 100% (or 1.0).

Step 2: Determine total grams from each macronutrient

Now we simply divide the total calories from each macronutrient by the caloric content of each macronutrient. This tells us how many grams of each food our guy will be eating each day.

Carbohydrate: 1620 calories / 4 cal/gram =405 grams carbohydrate

Protein: 540 cal / 4 cal/gram = 135 grams protein

Fat = 540 calories / 9 cal/gram = 60 grams fat per day

So this particular diet, with 2700 calories and 60% carbs, 20% protein and 20% fat yields a diet of 405 grams of carbohydate, 135 grams of protein and 60 grams of fat per day. For the remainder of the diet setup, you’d divide that up across some number of meals including pre- and post-workout, all that jazz.

Operation 2:
Working Backwards Part 1: Determining Diet Composition

You can just as easily work the math backwards, to determine what percentage of each nutrient a given diet is. Let’s say someone was eating 150 grams of protein, 200 grams of carbohydrate, and 50 grams of fat and we want to find out how many total calories they are eating and what the percentages of the diet are.

Step 1: Determine caloric intake

First you simply mutiply the total grams of each nutrient by the caloric content of that nutrient. That tells you how many calories they are eating each day

Protein: 150 grams * 4 cal/gram = 600 calories from protein

Carbs: 200 grams * 4 cal/gram = 800 calories from carbs

Fat: 50 grams * 9 cal/gram = 450 calories from fat

From those values, you can calculate total daily caloric intake by simply adding up the numbers.

Total calories = 600 + 800 + 450 = 1850 calories per day.

Step 2: Determine percentage from each nutrient

Now simply divide the calories from each nutrient by the total number of calories being consumed to determine the percentage each nutrient is providing. Multiply the decimal amount by 100 to get the percentage

Protein: 600 calories/1850 calories = 0.32 * 100 = 32%

Carbs: 800/1850 = 0.43 * 100 = 43%

Fat: 450/1850 = 0.24 * 100 = 24%.

So our example person is consuming 1850 calories per day with 32% from protein, 43% from carbs and 24% from fat.

You can use the identical math above to determine the composition of a given food (based on the food label) or a given meal.

So say you wanted to determine the macronutrient percentages on a food or a meal that contained 10 grams of protein, 20 grams of carbohydrates, and 9 grams of fat.

Step 1: Determine calories from each nutrient

First you’d simply multiply the total grams of each nutrient by the caloric content of that nutrient.

Protein: 10 grams * 4 cal/gram = 40 calories

Carbohydrate: 20 grams * 4 cal/gram = 80 calories

Fat: 9 grams * 9 cal/gram = 81 cal

Although most food labels list the total caloric content, even if they don’t, you can easy figure it out by adding up the totals above. This food/meal would contain 201 calories (40 cal + 80 cal + 81 cal)

Step 2: Determine percentages from each nutrient

Now you simply divide the total calories from each nutrient by the total calories in the food.

So this food or meal would contain 201 calories, with 20% protein, 40% carbs and 40% fat. Whether those percentages mean anything is the topic of the next chapter

A note on food labels

Many people become perplexed when they do the math above on food labels and find that the caloric content listed isn’t the same as what they calculate. So you might see a food that was listed as containing 212 calories with 10 grams protein, 20 grams of carbs and 9 grams of fat (which, as above, only yields 201 calories). There are a couple of reasons that this happens.

The first is that determining the caloric content of a given food isn’t doesn’t give perfect values, there is always a little bit of slop. As well, the 4, 9 and 4 cal/g values are rounded values in the first place. Finally, food labels almost always round off the values for protein, carbs and fat grams (for example, a food containing less than 0.5 g of fat can list it as 0 grams of fat). If the food listed above actually contained 10.5 grams of protein (44 calories), 20.5 grams of carbs (84 calories) and 9.5 grams of fat (85 calories), that would make up for the difference in values.

Ultimately, these types of tiny differences are no big deal. Even under the best circumstances, caloric estimates are only estimates and there’s always going to be a little bit of slop either direction. We’re not doing clinical nutrition here and, as long as it’s not excessive, small discrepancies in calore values are nothing worth worrying about.

Comments

One Response to “Diet Percentages: Part 1”

melissa on
March 13th, 2009 7:33 pm

Having trouble trying to get help from here its not working the question i need is multiply your total grams of fat by 9 and total grams carbs by 4 separtate calculations) this give you your total calories from fat and carbs, then divide the results by your total calorie intake and multiply by 100 this is the percentage of calories you consumed of fat and carbs, my carbs is 268 fat is 82 my isnt comming out right can you help.. Analze your protein intake by first calculating your personal RDA based on your body weight. convert pounds to kilograms by dividing your in pounds by 2.2 then multipy your body weight in kg by 0.8 grams protein/kg of body weight.